Soybean aphid arrived in the United States around 2000 and quickly became a serious crop pest. University of Minnesota researchers have been studying whether a naturally introduced parasitoid wasp, Aphelinus certus, can help keep aphid populations below damaging levels.
How the wasp works
The female wasp lays an egg inside an aphid. The developing larva kills its host, leaving a dark, swollen ‘mummy’ from which a new adult wasp emerges. Although several specialised Asian parasitoids were deliberately evaluated for biological control, they did not establish successfully. A. certus, a more generalist species that was not approved for release, appeared accidentally in Pennsylvania and was detected in Minnesota by 2011.
Field observations and modelling suggest the wasp can contribute meaningful suppression under normal conditions, but it does not eliminate risk. The estimated chance of keeping aphids below an economic injury level was stronger than the chance of keeping them below the lower treatment threshold. Weather, landscape, aphid growth and other predators all affect the outcome.
Farm practice can protect the control system
Parasitoid mummies can survive at the soil surface but are less likely to emerge when buried. Reduced tillage may therefore help overwintering populations. Avoiding unnecessary broad-spectrum insecticides also protects parasitoids, lady beetles and other natural enemies.
Biological control does not replace scouting. Minnesota guidance still uses a threshold of about 250 aphids per plant with increasing populations during the susceptible crop stage. Identifying whether natural enemies are present can improve the decision, but treatment should follow local recommendations and current field conditions.
The research shows why pest management is an ecological process. Habitat and selective decisions can preserve free control, while regular monitoring protects yield when that control is not enough.
Read the University of Minnesota Extension summary, Strategic Farming session focused on biological pest control.